Joe Biden is rising ...

Vice President Joe Biden. REUTERS/Jonathan Ernst Vice President Joe Biden isn't even a presidential candidate yet, but he's surging in a new national poll of the Democratic primary race.

A new Monmouth University poll of national Democratic voters shows Biden — who is weighing a run— gaining on Democratic presidential front-runner Hillary Clinton. He is also slightly ahead of Sen. Bernie Sanders (I-Vermont) for the first time among likely Democratic voters.

According to Monmouth, Biden grabs 22% of the vote nationally among Democratic voters, compared with Sanders' 20%.

Both trail Clinton, who has 42% support, but both have steadily gained steam over the past several months.

"For a guy who is not running for president, Biden sure is making headway against the front-runner," Patrick Murray, the director of the Monmouth University Polling Institute, said.

"There also seems to be an opening with the more liberal Sanders voters if Biden plays his cards right."

Since announcing his candidacy earlier this year, meanwhile, Sanders has rocketed up polls, overtaking Clinton in several surveys of New Hampshire, the state with the earliest primary.

Biden has been watching Sanders' rise closely, and he told donors earlier this month that he was impressed by Sanders' ability to excite voters.

"I am not a populist. But Bernie Sanders, he's doing a hell of a job," Biden said, according to Politico. He made similar remarks Monday while participating in the Labor Day parade, where he avoided mention of Clinton.

Biden greeting the crowd gathered along the streets at a Labor Day parade on Monday in Pittsburgh. AP Photo/Keith Srakocic

Despite Biden's rising levels of support among Democrats, he will still have huge obstacles to overcome if he chooses to jump into the race.

Beyond the obvious problems of launching a campaign so late into the race, including a lack of a campaign organization and a huge fund-raising deficit compared with his rivals, Biden is also not viewed much more favorably among the Democratic base.

According to Tuesday's Monmouth poll, 71% of Democratic voters view both Biden and Clinton in a positive light, though more Democrats view Clinton unfavorably.

Biden, though, has a higher favorability rating among Republicans and independents. A Washington Post/ABC poll released earlier this month indicated that 26% of Republicans had a positive view of Biden, compared with 13% who had a positive view of Clinton.